1. Field of the Invention
The present invention relates to an apparatus for controlling cell transmission timing, for example, in an ATM (Asynchronous Transfer Mode) exchange.
2. Description of Prior Art
As one such exchange, Japanese patent laid-open publication No. 7-327033 has taught a method and apparatus for controlling cell transmission timing. FIG. 7 depicts the configuration of the cell transmission timing controller (hereinafter, referred to as a shaper) disclosed in the publication. The shaper accommodates a plurality of connections 7 each involving a plurality of cells 8 to be transmitted. To control cell transmission, the shaper incorporates the cell interval managing unit 1, the cell transmission time managing unit 2, the cell multiplexing unit 3, the connection managing unit 4, and the unit time generating unit 5, wherein the cell transmission time managing unit 2 includes the cell transmission time table 2A, the cell interval managing unit 1 includes the cell interval table 1A, and the cell multiplexing unit 3 includes the cell transmission scheduling table 3A. The cell transmission time table 2A stores times at which cells are transmitted. The cell interval table 1A stores cell intervals between the times at which a cell (hereinafter, referred to as "preceding cell") is transmitted and the times at which a cell following the preceding cell) (hereinafter, referred to as "following cell") is transmitted.
To obtain the time at which the following cell will be transmitted, the shaper reads out of the cell interval table 2A a cell interval with respect to the preceding cell and the following cell. Thereinafter, the shaper adds the read cell interval to the time at which the preceding cell will be transmitted or has been transmitted (hereinafter, referred to as a preceding cell transmission time), thus storing the preceding cell in the cell transmission time managing unit 2A. In this way, the shaper determines the time at which the following cell will be transmitted (hereinafter, referred to as a following cell transmission time.)
Here, a unit time [seconds/cell] is defined as the period of time which one cell requires for transmission. Assuming that a cell transmission rate of one of the connections 7 is R [cells/second] and that the cell transmission rate of the shaper is P [cells/second], the cell interval in the connection is given as P/R [seconds/cell]. This indicates that a smaller cell transmission rate of a connection permits the cell interval in the connection to be longer, and vice versa.
After reading out the following cell transmission time from the cell transmission time managing table 2A, the cell transmission time managing unit 2 registers the following cell in the cell transmission scheduling table 3A in such a fashion that the following cell can be transmitted at the following cell transmission time. Once the following cell transmission time comes, the following cell is permitted to be transmitted.
As described above, if the cell transmission rate of a connection 7 is small, the cell interval in the connection 7 becomes large. Since the following cell transmission time is calculated using the cell interval with respect the preceding cell and the following cell, the large cell interval makes the following cell transmission time large. This forces the cell interval table 1A, the cell transmission time table 2A and the cell transmission scheduling table 3A to be large enough to store such large values.
Herein, assuming that the cell transmission rate or bandwidth of the shaper is 622.08 [Megabits/second], namely, (622.08.times.10 6)/(8.times.53) [cells/second] and that the cell transmission rate R of a connection 7 is 10 [cells/second], the cell transmission scheduling table 3A requires a storage area corresponding to the cell interval more than 146,717 [seconds/cell].